test: add in a regression test for #8300

Closes #8300

Author: ckipp01

tests/pos/i8300.scala

@@ -0,0 +1,15 @@
+// https://github.com/lampepfl/dotty/issues/8300
+
+type Bar[X] = X match {
+  case List[a] => List[Tuple1[a]]
+  case Set[a]  => Set[Tuple1[a]]
+}
+
+object Test:
+  (Set(1, 2, 3), List("a", "b")).map(
+    [A] =>
+      (a: A) =>
+        a match {
+          case it: Iterable[x] => it.map(Tuple1(_)).asInstanceOf[Bar[A]]
+      }
+  )